Ionic penetration of vitreous objects

ABSTRACT

Vitreous objects such as glass and ceramics are strengthened by replacing medium sized ions by smaller ions and thereafter replacing the smaller ions by ions larger than the medium sized ions. The first step is conveniently by thermal ion exchange and the second by electrolytic ion exchange. A new apparatus forms a perfect seal for the electrolytic impaction.

United States Patent Tran et a1. N0v.'20, 1973 [54] IONIC PENETRATION 0FVITREOUS 3,174,919 3/1965 Spremulli 204/130 OBJECTS 1,832,607 11/1931Zworykin... 204/130 1,785,888 12/1930 Cox et al. 204/130 Inventors:Thach Lan Tran, (lesson; Alain 3,524,737 8/1970 Doyle etal. 65/30Bonnetin, Clichy-sous-Bois, both of 3,287,201 11/1966 Chisholm et al65/30 France FOREIGN PATENTS OR APPLICATIONS 1 Assigneel Saint-60min,Nellilly-sur-sienfi, 6,501,168 8/1965 Netherlands 65/30 France [22]Filed: Sept 9 9 0 Primary ExaminerI-loward S. Williams AssistantExaminer-R. L. Andrews [2]] Appl. No.: 70,784 Att0meyPennie, Edmonds,Morton, Taylor and Adams et a1. [30] Foreign Application Priority DataSept. 10, 1964 France 6930800 [57] ABSTRACT Vitreous objects such asglass and ceramics are [52] US. Cl. 65/30 strengthened by replacingmedium sized ions by [S1] C03c 21/00 'smaller ions and thereafterreplacing the smaller ions [58] Field of Search 204/130; 260/465 R; byions larger than the medium sized ions. The first 156/329, 315; 65/114,115, 30 step is conveniently by thermal ion exchange and the second byelectrolytic ion exchange. A new apparatus [56] References Cited forms aperfect seal for the electrolytic impaction.

UNITED STATES PATENTS 7 Claims, 2 Drawing Figures 3,486,995 12/1969Evers 204/130 3,218,220 11/1965 Weber 204/130 IONIC PENETRATION OFVITREOUS OBJECTS This invention relates to the ionic penetration ofvitreous objects such as plates and sheets of glass or ceramic. A systemis known by which a plate of glass can be strengthened by theelectrolytic substitution of larger for smaller. ions in the .glass.According to that process a-glass plate is arranged between anodic andcathodic baths of which the anode contains an ion of an element largerthan one contained in the glass. An electrical field is set up betweenan anode in the anode bath and a cathode in the cathode bath and thelarger ions travel from theanodic electrolyte into the glass plate,expelling the smaller ions into the cathode compartment and putting theouter layers of the glass, in which smaller ions have been replaced bylarger ions, under compression. A balancing of forces occurs, puttingthe center of the plate under tension stresses.

In order to carry out the process various apparatus have been tried buthave been faced with a common problem, the short circuiting of theelectrical field around the edge of the glass plate, which defeats theprocess, as it is essential for the field to travel through the plate,not around it. As the problem of ionically impacting a glass plate tostrengthen it is known, this invention will be described in itsapplication to that problem.

The invention relates particularly to improvements in the treatment ofglass and ceramic articles by ion exchange under the impulse of anelectrical field.

The impaction of the surface ofa glass or ceramic object byion exchangeunder the power of an electrical field is often applied to glass platesto strengthen them. In the process the plate serves as a diaphragmthrough which pass the lines of force of the field, requiring that theanode and cathode compartments be isolated from each other so as toconfine the field to passage through the plate.

It is an object of the invention to improve the process, to seal theanode and cathode compartments from each other, to confine theelectrical field to passage through the plate, and to improve thegeneral process and the product which is produced.

The objects of the invention are accomplished, generally speaking, by amethod of subjecting a vitreous object to ionic flow which comprisessealing the object onto the end ofan annular mating part which issubstantially impervious to ionic flow, covering each face of the objectwith an electrolytic bath containing an electrode, of which one bathcontains an element which is to flow ionically into the object, andpassing an electric field from electrode to electrode through the bathsand the object in a direction and with an intensity which establishesionic flow of the element into the object;- and by apparatus for theionic packing of a vitreous object which comprises an anode compartmentcontaining an anode, a cathode compartment containing a cathode, and awall between the cathode compartment and the anode compartmentconsisting essentially of the vitreous object, said object being sealedto a mating annular face on at least one said compartment, and both saidcompartments containing conductive liquids capable of transmitting ionsto and from the object under a field of force supplied by theelectrodes.

According to the invention the receptacles for the anode or cathode bathhas the form of an annulus the edge of which is conformed to a matingface on the object. In the case of a flat glass plate the edge or rim ofthe annulus orannular wall of the compartment lies in a plane and can bematched to a mating surface on the plate. In the case of a flat platethe rim of the compartment will be just slightly smaller incircumference but of the same shape. The edge of the receptacle, or themating face of the sheet, or both will be coated with an impermeableadhesive, impermeable both to field and to fluid, the mating faces willbe brought together and the adhesive will be hardened. This forms acompartment, a receptacle for the anode or cathode bath, with the plateto be treated as a diaphragm closing one end. By combining thisstructure with another electrode compartment of opposite sign theprocess of ion exchange can be carried out through the plate moreefficiently than before. The lines of force of the electric field passalmost exclusively through the glass plate, the two compartments beingrigorously isolated. According to one mode of operating the inventionthe first electrolytic compartment is composed of the plate and itsattached annulus and the second electrolytic compartment is composed ofan ordinary electrolytic cell of sufficient size to receive the firstcompartment within it. According to a second mode the second compartmentis also provided with an annular rim which matches a mating face on theglass plate so that the plate serves as a diaphragm between opposedcompartments of similar construction, both of which are sealed to it inthe same way but to opposite sides.

The drawings illustrate two forms of the invention, which 7 FIG. 1 is avertical section through an electrolytic apparatus according to theinvention;

FIG. 2 is a vertical section through a modification.

In the apparatus of FIG. 1 the anode compartment includes a receptacle 1which holds an electrolyte liquid 2. The cathode compartment includes anannulus 4 made of a dielectric, of material which is electricallyinsulating, the lower face 4a of which is mated to the periphery of theupper face of the glass plate 5. The mating faces of the annulus and theplate are sealed and bonded together by an adhesive 4b. Thus the annulusand plate constitute the cathode compartment which contains electrolyte6 and into which dips the cathode 8. The anode compartment 1 is largeenough to receive the cathode compartment, the electrolyte 2, and theanode 7. When the anode and cathode are supplied with adequateelectrolytic power an electric field is established which causes theions in electrolyte 2 to migrate into the glass and original ions to beexpelled from the glass into the cathode electrolyte.

It is to be observed that all of the plate 5 is exposed to the passageof the field and to ion exchange, the ions passing through the face 5aand through the vertical edges 5b.

In FIG. 2 the glass plate 5 is clasped between and sealed to matchingannular faces 20-20 of two shells 10, 11. The shells are mirror imagesof each other and have open tops through which the anode 7 and cathode 8enter the electrolytes 2 and 6. The annular faces 2 0-20 are sealed tomating faces of the glass by adhesive as described for FIG. 1.

In this form of the invention a narrow rim at the edge of the plate 5 isnot subjected to the passage of the electric field set up by theelectrodes and electrolytes.

-In FIG. 2 the shells may be of electrical insulating material orelectrically conductive, but in FIG. 1 the cathode compartment 4 must bemade of electrical insulating material.

As these apparatus and processes are to be used not only at low but athigh temperature, for instance at the temperature of molten salt baths,it is necessary that an adhesive capable of withstanding the conditionswhich are to be employed should be adopted. This presents no problem atlow temperature but a substantial problem at high temperature. It is anobject of the invention to provide adhesives which will be usefulthroughout the full range of usefulness of the process.

This object is accomplished by the use as an adhesive of thealkyl-aryl-silicone class of polymerizable resins. A preferred subclassof these resins for use at temperatures up to 450-460C. are the methylphenyl silicones. A preferred member of the subclass is methyl phenylsilicone in which the ratio c,H,:si is between 0.9 and 1.75 inclusive,and the ratio CH :Si is between 0.25 and 0.5, inclusive. These resinscan be used successfully below 460C. but above that temperature willchar. They are quite satisfactory at 450C.

To form the seals a silicone resin of this type is applied in the coldby any satisfactory means, for instance by brush or by burette, to oneor both mating faces, which may be preliminarily scoured with emery andcleaned. A film of about 0.2 to 0.3 mm. is adequate and eliminatesbubble formation during polymerization. An industrial resin called SISS805 can be used. After application of the resin, for instance to theedge of the shell, it is heated to about 230C. for ten minutes, andcooled to about 60C. at which temperature it is in a tacky adhesivestate affording a tight seal without being loose. On the other hand,ifit is cooled to room temperature after partial polymerization at 230C.it becomes hard and does not adhere well, in which case it can bereheated at the moment of use, or the mating faces can be reheated toimpart renewed powers of adhesion. The first of these techniques is thefaster.

It is sometimes useful to apply some resin over the joints after theparts have been mated to reinforce the seal and block off anyinadvertently formed fissures.

The resin, after polymerization and application to the mating facesforms a tight seal without further treatment; it becomes hard whencooled to room temperature and polymerizes to hardness when used in hotprocesses.

Tests have shown that such joints remain tight and satisfactory after 16hours of use at 430C. even if the shells become deformed. At 450C. theelectrical insulating powers of the resin are still satisfactory, and nochemical side reactions occur with the electrolytes, even when moltenpotassium nitrate is used in one or in both compartments at thistemperature.

Many high temperature refractories are known which are electricallynon-conductive and may be used for the construction of the apparatus ofFIG. 1. The apparatus of FIG. 2 can be made of any substance which willstand the temperature used, and not react with the electrolyte, asconductivity is of no importance.

In a modification of the apparatus of FIG. 2 the level of the liquidelectrolyte is lowered below the upper edge of the object and the upperhorizontal seal is omitted, but such structure runs the risk of shortcircuiting and the apparatus of FIG. 2 is preferred.

In general, with some of the usual electrolytes a current of about 6mA/cm. suffices. Among the salts used in a state of fusion are NaNO(3O6C.) and KNO (340C) As the conductibility of glass increases with thetemperature it is advisable to work at high temperature, which takesless power and gives better results. In general the ion exchangereaction is carried out at above 400C. and not above 450C. In this rangeis obtained the highest efficiency.

The objective of the ion exchange in glass plates is to replace smallions (Na, Li) by larger ions (K).

EXAMPLE I Silica-soda-lime glass of ordinary windshield type,composition SiO 71 percent by weight, A120 1% CaO 11%, MgO 3%, NaZO 14%,2 mm. thick, was mounted as in FIG. 1 and treated as follows:

The external container was aluminum.

The separation ring was Pyrex glass,

The electrode was non-polarizable platinum with a sleeve of poroussillimanite.

Temperature 430C. Current density 10 mA/cmf, corresponding to a voltageof 60 to 100.

Time of treatment: 15 minutes.

Thickness of K layer obtained, p.m.

Polarity was reversed for 9 minutes after which each face had a layer inwhich K had replaced Na to a depth of 55 um.

EXAMPLE 2 The electrochemical reinforcement was preceded by a purelythermal ion exchange at 600C. in a bath containing the eutectic LiSO/MnSO During this phase the surface sodium present in the glass wasreplaced by lithium without establishing strains other than those due tothermal constriction during cooling.

2a. Sodium ions were used to expel lithium ions during a period oftreatment lasting 10 minutes. The initial lithium impregnation was to adepth of 150 pm. A bath of KNO at 430C. and 4 mA/cm. was applied in theapparatus of Example I for 35 minutes. The anode face received K to adepth of about 75 pm. which established a compression state of 6,000bars. This compressed layer was followed by a tensioned state toward thecenter of the sheet which was at about 5,000 bars. The elimination oflithium from the opposite face established a compression stateapproximating a maximum of 4,000 bars.

2b. After subjecting one face of a glass plate to the sodium-lithiumexchange for 5 minutes, producing a lithic layer of about .Lm., theplate is reversed so that that layer becomes cathodic. After impositionof a current of 4 mA/cm. for 45 minutes at 400C. the lithium waseliminated and a compression state of about 4,000 bars was formed in alayer about 60 um. thick.

EXAMPLE 3 In this example a vitroceramic plate 2 mm. thick was treatedin the apparatus of FIG. 2. The piece had the composition SiO 56 percentby weight, M20 6%, C210 19%, Mg 04.5%, Na O 9.5%, F 5%. The two shellsof FIG. 2 were aluminum. The electrodes were platinum covered withfritted alumina. The electrolyte was molten KNO3- The treatment was at450C. under 3 mA/cm. for 10 minutes. Penetration by K into the testpiece was to a depth of 25 um.

As many apparently widely different embodiments of the present inventionmay be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments.

What is claimed is:

l. A method of ionically reinforcing a vitreous object which comprises,adhesively sealing the object along a line of seal onto the end of anannular mating part which is substantially impervious to ionic flow, bycoat ing the line of seal on either the mating part or the object,evenly with a polymerizable adhesive, polymerizing the adhesive to thetacky state, contacting the mating part and the object along the line ofseal, and polymerizing the adhesive to a state of solidity, coveringeach face of the object with discrete electrolytic baths, respectively,each containing a respective one of two electrodes, one said bathcontaining ions of an alkali metal which are to flow into thecorresponding face of the object, and passing an electric current fromone electrode to the other, through the baths and the object in adirection and with an intensity which effects flow of alkali metal ionsfrom said one bath into the face of the object contacted thereby.

2. A method according to claim 1 in which the polymerizable adhesive isan alkyl-aryl-silicone.

3. A method according to claim 2 in which the polymerizable adhesive ismethyl phenyl silicone.

4. A method according to claim 3 wherein the ratio C ll /Si is between0.9 and 1.75, and the ratio CH /Si is between 0.25 and 0.5.

5. A method according to claim 4 in which said one bath is a moltenpotassium salt melting at temperatures about 450 C. i

6. A method according to claim 1 in which the object is preliminarilysubjected to thermal ion exchange by contact with a bath consistingessentially of a mixture of lithium sulfate and manganese sulfate at atemperature of about 600 C.

7. A method of ionically reinforcing a vitreous object which comprisessealing to a first surface of the object a part having a closed line ofseal and forming with an area of the first surface a container forelectrolyte, by first applying to the object or the part along the lineof seal, an alkyl-aryl-silicone polymerizable adhesive, polymerizing theadhesive to the tacky state, contacting the object and part along theline of seal, and polymerizing the adhesive to solidity, the part beingimpervious to ionic flow, establishing in the container a first moltenelectrolytic bath containing ions of an alkali metal and covering saidarea, establishing a second molten electrolytic bath in contact with asecond area of a surface of the object opposite said first surface, andpassing an electric current from one electrolytic bath to the other andthrough the object, in a direction and with an intensity which effectsflow of alkali metal ions from the first bath into the area of theobject contacted thereby.

2. A method according to claim 1 in which the polymerizable adhesive isan alkyl-aryl-silicone.
 3. A method according to claim 2 in which thepolymerizable adhesive is methyl phenyl silicone.
 4. A method accordingto claim 3 wherein the ratio C6H5/Si is between 0.9 and 1.75, and theratio CH3/Si is between 0.25 and 0.5.
 5. A method according to claim 4in which said one bath is a molten potassium salt melting attemperatures about 450* C.
 6. A method according to claim 1 in which theobject is preliminarily subjected to thermal ion exchange by contactwith a bath consisting essentially of a mixture of lithium sulfate andmanganese sulfate at a temperature of about 600* C.
 7. A method ofionically reinforcing a vitreous object which comprises sealing to afirst surface of the object a part having a closed line of seal andforming with an area of the first surface a container for electrolyte,by first applying to the object or the part along the line of seal, analkyl-aryl-silicone polymerizable adhesive, polymerizing the adhesive tothe tacky state, contacting the object and part along the line of seal,and polymerizing the adhesive to solidity, the part being impervious toionic flow, establishing in the container a first molten electrolyticbath containing ions of an alkali metal and covering said area,establishing a second molten electrolytic bath in contact with a secondarea of a surface of the object opposite said first surface, and passingan electric current from one electrolytic bath to the other and throughthe object, in a direction and with an intensity which effects flow ofalkali metal ions from the first bath into the area of the objectcontacted thereby.